Currently, the market of electronic books (e-books) has been expanded. In addition to the supply market for providing or selling e-books, the market of terminals used to execute e-books has been increasing.
E-books are generally used as teaching materials in schools and institutions, for example. A lecturer and a learner may record additional information to content to be learned from an e-book using a tool such as a pen, a post-it, and a highlighter.